An element with atomic number of 89 (actinium) to 103. Usually applied to those above uranium - 93 up (also called transuranics). Actinides are radioactive and typically have long half-lives. They are therefore significant in wastes arising from nuclear fission, e.g. used fuel. They are fissionable in a fast reactor. Minor actinides are americium, curium and neptunium. (World Nuclear)

Activation product

A radioactive isotope of an element (e.g. in the steel of a reactor core) which has been created by neutron bombardment. (World Nuclear)


The number of disintegrations per unit time inside a radioactive source. Expressed in becquerels. (World Nuclear)


As Low As Reasonably Achievable, economic and social factors being taken into account. This is the optimisation principle of radiation protection. (World Nuclear)

Alpha particle

A positively-charged particle emitted from the nucleus of an atom during radioactive decay. Alpha particles are helium nuclei, with 2 protons and 2 neutrons. (World Nuclear)


A particle of matter which cannot be broken up by chemical means. Atoms have a nucleus consisting of positively-charged protons and uncharged neutrons of almost the same mass. The positive charges on the protons are balanced by a number of negatively-charged electrons in motion around the nucleus. (World Nuclear)

Background radiation

The naturally-occurring ionising radiation which every person is exposed to, arising from the earth's crust (including radon) and from cosmic radiation. (World Nuclear)


See Cross section. (World Nuclear)

Base load

That part of electricity demand which is continuous, and does not vary over a 24-hour period. Approximately equivalent to the minimum daily load. (World Nuclear)


The SI unit of intrinsic radioactivity in a material. One Bq indicates one disintegration per second and is thus the activity of a quantity of radioactive material which averages one decay per second. (In practice, GBq or TBq are the common units). (World Nuclear)

Beta particle

A particle emitted from an atom during radioactive decay. Beta particles are generally electrons (with negative charge) but may be positrons. (World Nuclear)

Biological shield

A mass of absorbing material (e.g. thick concrete walls) placed around a reactor or radioactive material to reduce the radiation (especially neutrons and gamma rays respectively) to a level safe for humans. (World Nuclear)

Boiling water reactor (BWR)

A nuclear reactor in which water is boiled in the reactor vessel; the resulting steam drives a turbine to generate electricity (Westinghouse)


To form fissile nuclei, usually as a result of neutron capture, possibly followed by radioactive decay. (World Nuclear)

Breeder reactor

See Fast Breeder Reactor and Fast Neutron Reactor. (World Nuclear)


The process of undergoing fission (analogous to burning a fossil fuel) or otherwise becoming denatured in the reactor core. (World Nuclear)

Burnable absorber, burnable poison

A neutron absorber included in the fuel which progressively disappears and compensates for the loss of reactivity as the fuel is consumed. Gadolinium is commonly used, and erbium may also be used. (World Nuclear)


Measure of thermal energy released by nuclear fuel relative to its mass, typically Gigawatt days per tonne of fuel (GWd/t). (World Nuclear)


(in a PHWR reactor) A cylindrical reactor vessel which contains the heavy water moderator. It is penetrated from end to end by hundreds of calandria tubes which accommodate the pressure tubes containing the fuel and coolant. (World Nuclear)


Canadian Deuterium Uranium reactor, moderated and cooled by heavy water (except for the ACR design, which is cooled by light water). These are the most common PHWRs (cf heavy water reactor). (World Nuclear)


A cylinder spinning at high speed to physically separate gas components of slightly different mass, e.g. uranium hexafluoride with U-235 and U-238 atoms. (World Nuclear)

Chain reaction

A reaction that stimulates its own repetition, in particular where the neutrons originating from nuclear fission cause an ongoing series of fission reactions. (World Nuclear)


Tubing, usually an alloy of zirconium, which encapsulates nuclear fuel pellets in a reactor core (Westinghouse)


See Uranium oxide concentrate (U3O8). (World Nuclear)

Containment Building

Houses the reactor, pressurizer, reactor coolant pumps, steam generators, and other equipment or piping containing reactor coolant. (Westinghouse)

Control rods

A device in the core of a reactor which absorbs neutrons, and is used to control the rate of fission and to stop the chain reaction (Westinghouse)


Chemical process turning U3O8 into UF6 preparatory to enrichment. (World Nuclear)

Conversion ratio

(In a nuclear reactor) the ratio of new fissile nuclei to fissioned nuclei. In a normal reactor this as around 0.6, at high neutron energy in a fast reactor it may exceed 1.0, due to more neutrons per fission, making it a net breeder. (Above about 0.1 MeV, ie fast neutron spectrum, the number of neutrons released per fission increases from around 2.5, and at 10 MeV it reaches about 4.) (World Nuclear)


A fluid, usually water, used to cool a nuclear reactor and transfer heat energy. The water also moderates, or slows down, the fission of neutrons (Westinghouse)


The central part of a nuclear reactor containing the fuel elements and any moderator. (World Nuclear)

Critical mass

The smallest mass of fissile material that will support a self-sustaining chain reaction under specified conditions. (World Nuclear)


Condition of being able to sustain a nuclear chain reaction. (World Nuclear)

Cross section

See neutron cross-section (World Nuclear)


Disintegration of atomic nuclei resulting in the emission of alpha or beta particles (usually with gamma radiation). Also the exponential decrease in radioactivity of a material as nuclear disintegrations take place and more stable nuclei are formed. (World Nuclear)


Removal of a facility (eg reactor) from service, also the subsequent actions of safe storage, dismantling and making the site available for unrestricted use. (World Nuclear)


The chemical process of turning UF6 into uranium oxide. Typically depleted UF6 may be processed for long-term storage in a more stable chemical form. HF is a by-product. (World Nuclear)

Delayed neutrons

Neutrons released by fission products up to several seconds after fission. These enable control of the fission in a nuclear reactor. (World Nuclear)

Depleted uranium

Uranium having less than the natural 0.7% U-235. As a by-product of enrichment in the fuel cycle it generally has 0.25-0.30% U-235, the rest being U-238. Can be blended with highly-enriched uranium (eg from weapons) to make reactor fuel. (World Nuclear)


"Heavy hydrogen", a stable isotope having one proton and one neutron in the nucleus. It occurs in nature as 1 atom to 6500 atoms of normal hydrogen, (Hydrogen atoms contain one proton and no neutrons). (World Nuclear)


Natural change in the nucleus of a radioactive isotope as particles are emitted (usually with gamma rays), making it a different element. (World Nuclear)


The energy absorbed by tissue from ionising radiation. One gray is one joule per kg, but this is adjusted for the effect of different kinds of radiation, and thus the sievert is the unit of dose equivalent used in setting exposure standards. (World Nuclear)

Electron volt

1.6 x 10-19 joules, the amount of kinetic energy gained by a single electron when it accelerates through an electrostatic potential difference of one volt. (World Nuclear)


A chemical substance that cannot be divided into simpler substances by chemical means; atomic species with same number of protons (being the atomic number of the element). (World Nuclear)

Enriched uranium

Uranium in which the proportion of U-235 (to U-238) has been increased above the natural 0.7%. Reactor-grade uranium is usually enriched to about 3.5% U-235, weapons-grade uranium is more than 90% U-235. (World Nuclear)


Physical process of increasing the proportion of U-235 to U-238. See also SWU. (World Nuclear)

Fast breeder reactor (FBR)

A fast neutron reactor (q.v.) configured to produce more fissile material than it consumes, using fertile material such as depleted uranium in a blanket around the core. (World Nuclear)

Fast neutron

Neutron released during fission, travelling at very high velocity (20,000 km/s) and having high energy (c 2 MeV). (World Nuclear)

Fast neutron reactor

A reactor with no moderator and hence utilising fast neutrons. It normally burns plutonium while producing fissile isotopes in fertile material such as depleted uranium (or thorium). (World Nuclear)

Fertile (of an isotope)

Capable of becoming fissile, by capturing neutrons, possibly followed by radioactive decay; eg U-238, Pu-240. (World Nuclear)

Fissile (of an isotope)

Capable of capturing a slow (thermal) neutron and undergoing nuclear fission, e.g. U-235, U-233, Pu-239. (World Nuclear)


The splitting of a heavy nucleus into two, accompanied by the release of a relatively large amount of energy and usually one or more neutrons. It may be spontaneous but usually is due to a nucleus absorbing a neutron and thus becoming unstable. (World Nuclear)

Fission products

'Daughter' nuclei resulting either from the fission of heavy elements such as uranium, or the radioactive decay of those primary daughters. Usually highly radioactive. (World Nuclear)

Fissionable (of an isotope)

Capable of undergoing fission: If fissile, by slow neutrons; otherwise, by fast neutrons. (World Nuclear)

Fossil fuel

A fuel based on carbon presumed to be originally from living matter, eg coal, oil, gas. Burned with oxygen to yield energy. (World Nuclear)

Fuel assembly

Structured collection of fuel rods or elements, the unit of fuel in a reactor. (World Nuclear)

Fuel Cycle

The sequence of steps involved in supplying, using, and disposing of the fuel used in nuclear reactors (Westinghouse)

Fuel fabrication

Making reactor fuel assemblies, usually from sintered UO2 pellets which are inserted into zircalloy tubes, comprising the fuel rods or elements. (World Nuclear)

Gamma rays

High energy electro-magnetic radiation from the atomic nucleus, virtually identical to X-rays. Genetic mutation: Sudden change in the chromosomal DNA of an individual gene. It may produce inherited changes in descendants. Mutation in some organisms can be made more frequent by irradiation (though this has never been demonstrated in humans). (World Nuclear)


One billion units (eg one gigawatt is 109 watts or one million kW). (World Nuclear)


Crystalline carbon used in very pure form as a moderator, principally in gas-cooled reactors, but also in Soviet-designed RBMK reactors. (World Nuclear)

Gray (Gy)

The SI unit of absorbed radiation dose, one joule per kilogram of tissue. (World Nuclear)

Greenhouse gases

Radiative gases in the Earth's atmosphere which absorb long-wave heat radiation from the Earth's surface and re-radiate it, thereby warming the Earth. Carbon dioxide, methane and water vapour are the main ones. (World Nuclear)


The period required for half of the atoms of a particular radioactive isotope to decay and become an isotope of another element. (World Nuclear)

Heavy water

Water containing an elevated concentration of molecules with deuterium ("heavy hydrogen") atoms. (World Nuclear)

Heavy water reactor (HWR)

A reactor which uses heavy water as its moderator, eg Canadian CANDU (q.v.) which is a pressurised HWR (PHWR). (World Nuclear)

High-level wastes (HLW)

Extremely radioactive fission products and transuranic elements (usually other than plutonium) in used nuclear fuel. They may be separated by reprocessing the used fuel, or the spent fuel containing those isotopes may be regarded as high-level waste. HLW requires both shielding and cooling. High-enriched uranium (HEU): Uranium enriched to 20% U-235 or more. (That in weapons is about 90% U-235.) (World Nuclear)

In situ leaching (ISL)

The recovery by chemical leaching of minerals from porous orebodies without physical excavation. Also known as in situ recovery (ISR) or solution mining.

Intermediate-level waste (ILW)

Radioactive waste which requires shielding to protect people nearby, but not cooling. (World Nuclear)


An atom or molecule that is electrically-charged because of loss or gain of electrons.

Ionising radiation

Radiation (including alpha particles) capable of breaking chemical bonds, thus causing ionisation of the matter through which it passes and damage to living tissue. (World Nuclear)


Subject material to ionising radiation. Irradiated reactor fuel and components have been subject to neutron irradiation and hence become radioactive themselves. (World Nuclear)


An atomic form of an element having a particular number of neutrons. Different isotopes of an element have the same number of protons but different numbers of neutrons and hence different atomic masses, eg. U-235, U-238. Some isotopes are unstable and decay (qv) to form isotopes of other elements. (World Nuclear)

Laser enrichment

Uranium enrichment using tuned laser beams to cause photo-dissociation of UF6 to solid UF5+ so that the ionised UF5 (with U-235) can be separated. (World Nuclear)

Light water

Ordinary water (H20) as distinct from heavy water. (World Nuclear)

Light water reactor (LWR)

A common nuclear reactor cooled and usually moderated by ordinary water. It is s generic designation including BWR and PWR types. (World Nuclear)

Low-enriched uranium (LEU)

Uranium enriched to less than 20% U-235. (That in power reactors is usually 3.5 - 5.0% U-235.) (World Nuclear)

Low-level waste (LLW)

Radioactive waste which can be handled safely without shielding. (World Nuclear)

Low-level wastes

Mildly radioactive material usually disposed of by incineration and burial. (World Nuclear)

Metal fuels

Fuels using natural uranium metal, as used in a gas-cooled reactor. (World Nuclear)


One millionth of a unit (eg microsievert is 10-6 Sv). (World Nuclear)


Process by which minerals are extracted from ore, usually at the mine site, to produce a mineral concentrate for sale. (World Nuclear)

Mixed oxide fuel (MOX)

Reactor fuel which consists of both uranium and plutonium oxides, usually about 5% Pu, which is the main fissile component. (World Nuclear)


A material such as light or heavy water or graphite used in a reactor to slow down fast neutrons by collision with lighter nuclei so as to expedite further fission. (World Nuclear)

Natural uranium

Uranium with an isotopic composition as found in nature, containing 99.3% U-238, 0.7% U-235 and a trace of U-234. It can be used as fuel in heavy water-moderated or graphite-moderated reactors. (World Nuclear)


An uncharged elementary particle found in the nucleus of every atom except hydrogen. Solitary mobile neutrons travelling at various speeds originate from fission reactions. Slow (thermal) neutrons can in turn readily cause fission in nuclei of "fissile" isotopes, e.g. U-235, Pu-239, U-233; and fast neutrons can cause fission in nuclei of "fertile" isotopes such as U-238, Pu-239. Sometimes atomic nuclei simply capture neutrons. (World Nuclear)

Neutron cross-section

An indication of the probability of an interaction between a particle and a target nucleus, expressed in barns (1 barn = 10-24 cm2). (World Nuclear)

Nuclear reactor

A device in which a nuclear fission chain reaction occurs under controlled conditions so that the heat yield can be harnessed or the neutron beams utilised. All commercial reactors are thermal reactors, using a moderator to slow down the neutrons. (World Nuclear)


Elemental matter made up of atoms with identical nuclei, therefore with the same atomic number and the same mass number (equal to the sum of the number of protons and neutrons). (World Nuclear)

Oxide fuels

Enriched or natural uranium in the form of the oxide UO2, used in many types of reactor. (World Nuclear)


A transuranic element, formed in a nuclear reactor by neutron capture. It has several isotopes, some of which are fissile and some of which undergo spontaneous fission, releasing neutrons. Weapons-grade plutonium is produced in special reactors to give >90% Pu-239, reactor-grade plutonium contains about 30% non-fissile isotopes. About one third of the energy in a light water reactor comes from the fission of Pu-239, and this is the main isotope of value recovered from reprocessing used fuel. (World Nuclear)

Pressurised water reactor (PWR)

The most common type of light water reactor (LWR), it uses water at very high pressure in a primary circuit and steam is formed in a secondary circuit. (World Nuclear)


A high-strength tank containing steam and water used to control the pressure of the reactor coolant in the primary loop (Westinghouse)

Primary Loop

A closed system which provides cooling water to the reactor and transfers heat to the secondary loop. (Westinghouse)


The emission and propagation of energy by means of electromagnetic waves or particles. (cf ionising radiation) (World Nuclear)


The spontaneous decay of an unstable atomic nucleus, giving rise to the emission of radiation. (World Nuclear)


A radioactive isotope of an element. (World Nuclear)


The adverse health effect of a radionuclide due to its radioactivity. (World Nuclear)


A radioactive decay product of uranium often found in uranium ore. It has several radioactive isotopes. Radium-226 decays to radon-222. (World Nuclear)

Radon (Rn)

A heavy radioactive gas given off by rocks containing radium (or thorium). Rn-222 is the main isotope, from decay of radium-226. (World Nuclear)

Radon daughters

Short-lived decay products of radon-222 (Po-218, Pb-214, Bi-214, Po-214). (World Nuclear)

Reactor Coolant Pump

A piece of equipment designed to move the coolant through the primary loop so that the heat generated in the core can be transferred to the steam generator. (Westinghouse)

Reactor pressure vessel

A cylindrical steel vessel that contains the core, control rods, coolant, and the structures that support the core (Westinghouse)


A permanent disposal place for radioactive wastes.


Chemical treatment of used reactor fuel to separate uranium and plutonium and possibly transuranic elements from the small quantity of fission products, leaving a much reduced quantity of high-level waste. (cf Waste, HLW). (World Nuclear)

Secondary Loop

A system of piping that carries non-radioactive water, which then absorbs heat through steam generator tubes, is boiled and, as steam, is used to spin the turbines. (Westinghouse)

Separative Work Unit (SWU)

This is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, ie the extent of increase in the concentration of the U-235 isotope relative to the remainder. The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. (World Nuclear)

Sievert (Sv)

Unit indicating the biological damage caused by radiation dose measured in Gray (q.v.). One Gray of beta or gamma radiation absorbed has 1 Sv of biological effect; 1 Gy of alpha radiation has 20 Sv effect and 1 Gy of neutrons has 10 Sv effect. cf Dose. (World Nuclear)


The abrasion and removal of fragments of a target which is bombarded by protons in an accelerator. The fragments may be protons, neutrons or other light particles. (World Nuclear)

Spent fuel

Used fuel assemblies removed from a reactor after several years use and treated as waste. Often it is another term for used fuel. (World Nuclear)


Incapable of spontaneous radioactive decay. (World Nuclear)


Ground rock remaining after particular ore minerals (e.g. uranium oxides) are extracted. (World Nuclear)


Depleted uranium (cf. enriched uranium), with about 0.2 to 0.3% U-235.

Temperature coefficient of reactivity

The change in reactivity due to changed temperature in the fuel. A negative temperature coefficient means that the reactivity is diminished as the temperature rises, so as to reduce the rate of fission and hence reduce the temperature - a natural negative feedback. Fast reactors have a strong negative temperature coefficient which is the basis of automatic power regulation and even load-following. (World Nuclear)

Thermal reactor

A reactor in which the fission chain reaction is sustained primarily by slow neutrons, and hence requiring a moderator (as distinct from Fast Neutron Reactor). (World Nuclear)


Changing atoms of one element into those of another by neutron bombardment, causing neutron capture and/or fission. In an ordinary reactor neutron capture is the main event, in a fast reactor fission is more common and therefore it is best for dealing with actinides. Fission product transmutation is by neutron capture. (World Nuclear)

Transuranic element

A very heavy element formed artificially by neutron capture and possibly subsequent beta decay(s). Has a higher atomic number than uranium (92). All are radioactive. Neptunium, plutonium, americium and curium are the best-known

Uranium (U)

A mildly radioactive element with two isotopes which are fissile (U-235 and U-233) and two which are fertile (U-238 and U-234). Uranium is the basic fuel of nuclear energy. (World Nuclear)

Uranium hexafluoride (UF6)

A compound of uranium which is a gas above 56°C and is thus a suitable form in which to enrich the uranium. (World Nuclear)

Uranium oxide concentrate (U3O8)

The mixture of uranium oxides produced after milling uranium ore from a mine. Sometimes loosely called yellowcake. It is khaki in colour and is usually represented by the empirical formula U3O8. Uranium is normally sold in this form. (World Nuclear)

Used fuel

Fuel assemblies removed from a reactor after several years' use. (World Nuclear)


The incorporation of high-level wastes into borosilicate glass, to make up about 14% of it by mass. It is designed to immobilise radionuclides in an insoluble matrix ready for disposal. (World Nuclear)

Void coefficient of reactivity

The change in reactivity due to boiling of coolant or moderator in the actual core. A negative void coefficient means that the reactivity is diminished, and the balance of the chain reaction is affected so as to reduce the rate of fission and hence reduce the temperature - a natural negative feedback. (World Nuclear)


High-level waste (HLW) is highly radioactive material arising from nuclear fission. It can be what is left over from reprocessing used fuel, though some countries regard spent fuel itself as HLW. It requires very careful handling, storage and disposal. Intermediate-level waste (ILW) comprises a range of materials from reprocessing and decommissioning. It is sufficiently radioactive to require shielding and is disposed of in engineered facilities underground. Low-level waste (LLW) is mildly radioactive material usually disposed of by incineration and burial. (World Nuclear)


Ammonium diuranate, the penultimate uranium compound in U3O8 production, but the form in which mine product was sold until about 1970. See also Uranium oxide concentrate (World Nuclear)


Zirconium alloy used as a tube to contain uranium oxide fuel pellets in a fuel rod (part of a reactor fuel assembly). (World Nuclear)